That's technically true, but really not important. Houses are defined as vacant if they're unoccupied on the day of a census. There's many reasons a house might be technically vacant, but not currently be able to house a homeless person.

Was the house just sold, and is it unoccupied for a week or a month between owners? It's vacant. Did the owner just move into hospice or a memory care unit and their children haven't yet sold the house because they need to arrange an estate sale? It's vacant. Is the house under construction but is mostly built? It's vacant. Is it not safe to live in, but not officially condemned? It's vacant.

Want to move to a city? Either you have to find the apartment of someone moving out, or you have to move into a vacant unit.

Having a good number of vacant homes is a good thing, actually; having low numbers of vacancies in an area leads to housing becoming more expensive because you can't move into a unit that isn't vacant. Increasing housing supply relative to population leads to higher vacancy rates, but decreases housing costs.

Housing-first approaches to homelessness seem to be good in practice. But those are typically done by either government-built housing or government- subsidized housing; it's mostly orthogonal to vacancy rates.